1. Field of the Invention
The present invention relates to a method and system for improving performance of data transmissions on a local area network, and more particularly to the implementation of logic for creating sub-tokens on a dual ring token ring network.
2. Description of the Related Art
A local area network (LAN) is a collection of interconnected data processing resources, such as computers and printers, that may communicate and share each other's resources. The designation of "local" is a relative term, but a typical implementation of a LAN spans a single building or a group of neighboring buildings. Design choices for implementing a local area network include physical considerations such as type of media (e.g. coaxial cable, twisted pair, or fiber optics) and physical topology (e.g. bus, tree, star, or ring). Additionally, there are logical considerations which include access control and logical topology. The logical topology refers to how information is passed on the network. Access control is a broad term that generally refers to the control of data traffic on the network and typically is achieved by the use of one or more protocols.
The implemented protocol determines which stations have priority to access the media to send data and often depends on the physical and logical topology. On a bus topology, such as the one implemented by the Ethernet product (IEEE 802.3 protocol), access to the media is on an "as available basis" and software techniques are employed to manage media access. Such techniques include "data carrier detection" to determine if the media is busy, and "collision avoidance" to prevent garbled messages from two or more stations transmitting simultaneously.
Collisions are avoided in a ring topology implemented by the token ring product (IEEE 802.5 protocol), because a single "token" is used to gain access to the media. The token is a control signal, that circulates from one station to another until it is "seized" by a station that wishes to send data. Since a station can not send data without gaining control of the token, collisions are inherently avoided. The station seizing the token is known as the "sender" and a station designated as the destination is known as the "receiver." The sender station is said to have entered "transmit mode"--a mode only available to the token holder with the prior art protocols. All of the other stations on the ring, including the receiver station are in "listen mode".
The information circulates from one station to the next around the ring. Each bit arriving at a station's interface is copied into a 1-bit buffer and then copied back onto the ring, introducing a 1-bit delay time for each interface. While the information is in the buffer, each bit can be inspected. During this inspection, the receiver can recognize its address and copies all of the data designated as a "data frame" from the interface to the station's data bus. The sender will typically require several transmissions to send all of its intended message, depending on factors such as bandwidth (e.g., IBM provides a coaxial cable based token ring product that transmits 7 at 4 Megabits per second and also at 16 Megabits per second). Typically, the first bit of a "full frame" will circulate all the way around the ring and return to the sender before the entire frame has been transmitted. The full frame comprises the data frame and also other frames, including source and destination address. The sender is responsible for stripping each bit of data from the ring as it returns.
A station may hold the token for a predetermined "token holding" time. Typically this time is 10 msec, and is set at installation. If there is enough time left after the first frame has been transmitted then the sender may send more frames. If more time is needed to send additional frames, then the sender regenerates a 3 byte token frame, which it also seizes. If the last frame has been sent, a new token is regenerated but released onto the ring.
One particularly fast token ring network, which uses optical fiber, is known as the fiber distributed data interface (FDDI) and has a bandwidth of 100 Megabits per second. The basic FDDI protocol is closely modeled on the 802.5 protocol, but accounts for the increased capability of optical fiber. Optical FDDI networks are capable of supporting up to 1000 stations spanning a distance of 200 kilometers. If the 802.5 protocol were not deviated from, there would be a considerable time penalty introduced because new data frames could not be sent until a previous data frame had traveled all the way around the ring, back to the sender. Therefore, the FDDI protocol allows a sender to regenerate a new token as soon as it sends its last frame, without waiting for the frame to circulate to every station. But in most other ways, FDDI permits data structures very similar to 802.5, including acknowledgement bits in a frame status byte to indicate that data has been received.
There are two principal types of FDDI rings, a class A configuration and a class B configuration. The class B configuration employs a single ring, while the class A configuration employs a dual ring structure. In class A configurations, the primary ring is used for normal data transmissions and the secondary ring is a backup ring that is only used if there are problems with transmitting on the primary ring. The ring is automatically reconfigured in the event of a failure on the primary ring.
Clearly, it would be an advancement to the art to be able to employ both rings for concurrent data transmissions without degrading the reliability of the class A configuration. Further, it would be advantageous to employ the two rings for simultaneous transmissions without having to isolate the two rings from each other to avoid data collisions.